1. Field of the Invention
The present invention relates to a read channel for an optical disc drive, a magnetic hard disc drive, or the like.
2. Description of the Related Art
One noteworthy feature of optical disc drive units and discs in conformity with DVD (digital versatile disc), Blu-ray Disc (hereinafter referred to simply as “BD”) and other standards is that recording media are interchangeable and thus the discs are interchangeable among different models for writing and reading. However, situations can, in actuality, possibly arise where reading is extremely difficult due to the discs becoming damaged or soiled on their information-bearing surfaces. Although the scope of application of the present invention is not limited to a BD, description will hereinafter be given assuming that the present invention is applied to the BD. Likewise, terms as employed herein are basically those used for the BD.
FIG. 2 shows an example of the configuration of a read channel in a very basic form. A read signal processing system that leads an analog read signal through decoding to a bit stream, including an analog equalizer and a PLL (phase locked loop), is herein called “read channel.” The read signal processing system is herein assumed as a Viterbi decoding system that performs AD (analog-to-digital) conversion on the analog read signal and then performs signal processing on the signal. Accordingly, the term “read signal” is herein employed almost exclusively for a digital signal after the AD conversion. However, both the analog and digital signals, if apparent in context, are called merely “read signal” for the sake of simplicity, because those skilled in the art are unlikely to confuse between the analog signal before the AD conversion and the digital signal.
An analog read signal is equalized by an analog equalizer 1 and is then converted into a digital signal by an AD converter 2. At this point, the timing of sampling is determined by the channel clock.
After that, the signal is phase compared with the channel clock by a phase comparator 6. A phase error signal is smoothed through a loop filter 9 and is converted through a DA converter 11 into an analog signal, which is then inputted as a control voltage signal to a VCO (voltage controlled oscillator) 10. The VCO oscillates at the frequency indicated by the input voltage signal, and the oscillation frequency is used as the channel clock. In other words, the oscillation frequency is used as a drive clock for each element such as the AD converter, the phase comparator, the loop filter, the DA converter, and the Viterbi decoder 7. Since the fact that this closed loop forms a PLL and acts to bring the channel clock into synchronization with the clock of the read signal, and the details of operation thereof are well known in the art, detailed description thereof is not given. Also as for the Viterbi decoder, detailed description thereof is not given herein because the details of operation thereof do not have a direct relation to the present invention.
FIG. 3 is an illustration of assistance in explaining the principle of phase comparison. The phase are compared using edges (points where read signal cross the zero level), that is, points corresponding to boundaries between marks and spaces. The channel clock is in synchronization with the edges. The timing of AD conversion is shifted T/2 from clock timing referred to the edge (where T denotes a channel clock cycle). The read signal subjected to sampling after a lapse of T/2 from channel clock time nT will hereinafter be expressed as x(n) for the sake of simplicity (where n denotes an integer). In FIG. 3, the edge and sample points are shown by a dashed line and open (or white) circles, respectively, in an instance where the phase of the channel clock is in perfect synchronization with that of the edge. The edge lies at the time nT. At this time, the values of two sample points with the edge in between are defined as x(n−1) and x(n), respectively. The read signal is assumed to be in linear form in the vicinity of the edge. At this time, the relation between the values is as follows: x(n)=−x(n−1). In FIG. 3, the edge and sample points are shown by a solid line and solidly shaded (or black) circles, respectively, in an instance where the phase of the same edge lags ΔT behind that of the channel clock. The edge is assumed to lie between the channel clock times (n−1)T and nT, and the values of the sample points at the times (n−1)T and nT are defined as x(n−1) and x(n), respectively. Clearly, the relation between the values is as follows: x(n)≠−x(n−1). Obviously, assuming that the edge has linearity leads to Expression (1).ΔT∝x(n)−x(n−1)  (1)
In other words, detection of phase error can be accomplished by sampling the read signal by the channel clock and distinguish the edges, then the phase error can be determined from the difference in read signal levels between the two points with the edge in between.
When the phase error is determined from the signal level in the manner as mentioned above, the phase error cannot be accurately determined if an unwanted DC (direct current) component is superimposed on the read signal. Such a situation will be described with reference to FIG. 4. In FIG. 4, the edge and the sample points are shown by a dashed line and open (or white) circles, respectively, in an instance where the unwanted DC component is absent and the phase of the read signal is in perfect synchronization with that of the channel clock. The edge and the sample points are shown by a solid line and solidly shaded (or black) circles, respectively, in an instance where the phase of the read signal is in synchronization with that of the channel clock and the DC component is superimposed by Δx on the read signal. Even if the read signal is in synchronization with the channel clock, the superimposed DC component causes the phase comparison based on definition by Expression (1) to output an erroneous phase error value. For this reason, the DC component of the read signal is removed by use of a high-pass filter before entering the phase comparator. Even under this condition, however, a pattern-dependent DC component variation or the like remains in the read signal. The DC component variation depending on the pattern is removed by use of a DFB (duty feedback) slicer, which utilizes the fact that the bit stream recorded on an optical disc is modulated by using a modulation code that, if integrated at given or more intervals, exhibits the appearance possibility of “0” becoming equal to the appearance possibility of “1.” Since the DFB slicer is a technology well known in the art, detailed description thereof is not given.
Description will now be given with regard to a JFB (jitter feedback) DC compensator. When the DC component is zero and the PLL is completely locked, the phase error is zero, that is, the midpoint of the edge coincides with the zero level. When the read signal rather undergoes the DC variation with the PLL locked, the midpoint of the edge lies outside the zero level. Accordingly, the integration of the midpoint level of the edge can lead to a DC level. This method is employed provided that the PLL is locked, because the edge is used to detect the DC component.
PRML (partial response most-likely) decoding method involves decoding a read signal into a most-likely bit stream, while comparing the read signal at plural consecutive times with a target signal. Viterbi decoding method that is one of ML (most-likely) decoding methods is widely in practical use because of enabling a substantial reduction in the scale of the circuit. The PRML method has come into use also as a reading device for the optical disc in order to achieve higher speeds and larger capacities. Since the target signal is used provided that an unwanted DC component is completely absent, decoding performance undergoes deterioration if the DC component is superimposed on the read signal at the time of comparison of the read signal with the target signal.
When the device in a read mode, the device is made to minimize the occurrence of read error even if the disc is in bad condition such as situations where the disc is defective or soiled. For example, when the surface of the disc is so soiled that the read signal is almost entirely shielded, a defect detection technique such as described in Japanese Patent Application Publication Laid-open Application No. 2003-30850 can be used to minimize the influence of defects. It is well known in the art that the same or similar approach is used for the optical discs in general. In general outline, the approach involves a circuit that monitors the top envelope of the read signal and outputs a defect signal if the amplitude of the signal is equal to or less than a threshold for a given or longer time, as shown in FIG. 5. The approach involves, for the duration of output of the defect signal, holding the tracking, focusing and other controls and also holding the PLL of the read signal processing system and doing the like, thereby preventing undesirable operation resulting from the defects and hence minimizing the influence of the defects.
Besides local factors such as defects on the disc, a phenomenon occurs in which reading performance undergoes deterioration over an extremely wide range of the disc due to a disc's structure or the like, such as inter-layer interference of a dual layered disc. FIG. 6 shows an example of a read signal disturbed under the influence of the inter-layer interference. This is an instance where data is read from a layer-L1 of a rewritable dual layered Blu-ray Disc, that is, a layer close to the surface of the disc. As can be seen from FIG. 6, both the top and the bottom envelopes that should be, by nature, substantially flat undergo great disturbance under the influence of the inter-layer interference. During the reading of the data from the layer-L1, read light is focused on that layer. Part of the read light passes through the layer-L1 and reflected at layer-L0, and partially reaches a photodetector of an optical head. Since the light beams from the layer-L0 and the layer-L1 reach the photodetector simultaneously, the interference of the light beams occurs. Generally, a distance between the layer-L0 and the layer-L1 slightly varies according to their positions on the disc. When data is read from the disc under this condition, an interference pattern formed on the photodetector by the light beams from the layer-L0 and the layer-L1, changes with time. As a result, the read signal undergoes disturbance as shown in FIG. 6. When the disturbance is encountered in the signal as shown in FIG. 6, the signal recorded in the disturbed place cannot be accurately decoded and thus results in a burst error, as in the case of the defects. In the example shown in FIG. 6, the burst error is a few hundreds of bytes long. This length does not get in the way of reading in terms of the capability of an error correcting code of the Blu-ray Disc system. However, when data is read from a region where the distance between the layers varies greatly in the tangential direction of the disc, the state of the interference also changes more rapidly on the photodetector. Accordingly, the frequency of occurrence of disturbance of the signal as shown in FIG. 6 becomes higher, and thus the disturbance occurs plural times in one recording unit block (RUB). Under this condition, the probability of occurrence of the read error cannot be ignored. Incidentally, factors that cause the same or similar disturbance of the read signal include fingerprints and track deviation.